Lithium transition metal oxides, particularly those with a layered-type structure, such as LiCoO2, LiNiO2, LiMnO2 and LiVO2 and analogues thereof, are of interest as positive electrodes for rechargeable lithium batteries. The best-known electrode material, LiCoO2, is relatively expensive compared to the isostructural nickel and manganese-based compounds. Efforts are therefore being made to develop less costly electrodes, for example, by partially substituting the cobalt ions within LiCoO2 by nickel, such as in LiNi0.8Co0.2O2 or by exploiting a substituted system based on LiMnO2. Such layered compounds are sometimes stabilized by partially replacing the transition metal cations within the layers by other metal cations, either alone or in combination. For example, Li+ and/or Mg2+ ions may be introduced into the structure to improve the electronic conductivity of the electrode, or Al3+ or Ti4+ ions to improve the structural stability of the electrode at high levels of delithiation. Examples of such compounds are LiNi0.8Co0.5Al0.05O2 and LiNi0.75Co0.15Ti0.05Mg0.05O2.
Layered LiMO2 compounds containing either Co or Ni (or both) as the transition metal cations, M, with an average trivalent oxidation state, are oxidized during cell charging to a tetravalent oxidation state. Such compounds are highly oxidizing materials and can react with the electrolyte or release oxygen. These electrode materials can, therefore, suffer from structural instability in the charged state when, for example, more than 50% of the lithium is extracted from their structures. Although the layered manganese compound LiMnO2 has been successfully synthesized in the laboratory, it has been found that delithiation of the structure and subsequent cycling of the LixMnO2 electrode in electrochemical cells causes a transition from a layered MnO2 configuration to a 3-dimensional spinel-type [Mn2]O4 configuration. This transformation changes the voltage profile of the Li/LixMnO2 cell such that it delivers capacity over both a 4V and a 3V plateau. Other types of LiMnO2 structures exist, such as the orthorhombic-form, designated o-LiMnO2, in which sheets of MnO6 octahedra are staggered in zig-zig fashion unlike their arrangement in layered LiMnO2. However, o-LiMnO2 behaves in a similar way to layered LiMnO2 in lithium cells; it also converts to a spinel-like structure on electrochemical cycling.
Lithium-ion cells, which contain the LiMO2 electrodes described above are, in general, assembled in the discharged state to avoid safety problems and the inconvenience of handling charged electrode materials, such as lithiated graphite, and delithiated metal oxides, such as Li1−xCoO2 and Li1−xNiO2, which are highly reactive materials. However, a major disadvantage of lithium-ion cells is that all the lithium, which is transported between the positive and negative electrodes during charge and discharge is initially contained in the positive electrode, as in LiCoO2. On the initial charge of a graphite/electrolyte/LiCoO2 cell, some of the lithium that is deposited at the graphite electrode reacts with various chemical components in the cell: 1) the organic solvent of the electrolyte such as ethylene carbonate and dimethyl carbonate, 2) a component of the electrolyte salt such as the fluoride ion of LiPF6, and 3) a trace amount of water in the electrolyte. These reactions form a passive, protective layer on the lithiated graphite particles, thereby preventing further reaction between the lithiated graphite electrode and the electrolyte. Consequently, the lithium in the protective layer is unavailable for further electrochemical reaction, and cannot be transported back to the delithated Li1−xCoO2 electrode during discharge of the cell, which results in an irretrievable capacity loss from the lithium-ion cell. Therefore, it stands to reason that positive electrodes that contain an excess of lithium can be used to compensate for the electrochemically inactive lithium at the negative electrode, thereby combating the capacity loss of lithium-ion cells.
This invention describes a new class of electrochemically active compounds having the nominal formula Li2MO2, in which M represents two or more positively charged metal ions and in which there is twice as much lithium as in LiMO2 compounds, that can be used as electrodes to compensate for the capacity loss of conventional lithium-ion cells with electrodes, such as LiCoO2, LiNiO2, LiMn0.5Ni0.5O2 or the like. The surplus lithium in the electrode can also be used to ensure that there is always sufficient lithium in fully charged Li1−xMO2 electrodes to prevent cells from being overcharged to obtain a required capacity, thereby minimizing the degradation of the electrode structure by loss of oxygen or by oxidation of the electrolyte. The Li2MO2 compounds can also be used to compensate for the capacity loss at the negative electrode when other positive electrodes are used, such as Li1.03Mn1.97O4 spinel electrodes, or olivine-type electrodes such as LiFePO4. The compounds of the invention have additional functions for lithium cells, for example, they can be used as end-of-discharge indicators, or as negative electrodes in lithium cells. The invention extends to methods for synthesizing the Li2MO2 compounds.